1. Field of the Invention
The present invention pertains to light activated semiconductor switching devices, and particularly to thyristors which can be fired by a light-emitting diode.
2. Description of the Prior Art
An important design criterion of light activated thyristors is the ability to be fired (triggered) by means of a low intensity radiation source, as for example, a light-emitting diode (LED). A disadvantage associated with such use of LED's to fire thyristors is the relatively low density of carriers generated in the semiconductor body, which results in a slow firing rate. It is known that slow firing causes hot-spots in the semiconductor body due to high load current densities at random points where initial conduction sets in, thereby damaging the thyristor. The present invention, therefore, is addressed to the problem of making a light activated thyristor which responds more effectively to a low intensity radiation source.
Prior art thyristors are provided with base-emitter short circuits (shunts), which improve dv/dt response. Unfortunately, the shunts also drain off the light-generated carriers by providing low impedance paths to the emitter electrode directly from the area in the base where the carriers are generated. While it is true that a reduction in base-emitter shunting increases the current density of light-generating carriers passing through the base-emitter PN junction, such a reduction in shunting also adversely affects the dv/dt response. Thus, those skilled in the art will recognize that a reduction in shunting is an impractical trade-off, and that a means of increasing the current density of light-generated carriers crossing the base-emitter PN junction is required, while at the same time maintaining adequate shunting.